Here is an explication..no for the newbies....prepare to suffer...
In electricity, a conductor is a material like copper or silver which readily conducts electricity, while an insulator is the opposite: A material which doesn't conduct electricity well. In other words, a conductor has a very low resistance, and an insulator has a very high resistance. A semiconductor is a material that lies somewhere in between: It will sort of conduct electricity, but has a fair amount of resistance. Although not commonly cited as a "semiconductor", carbon exhibits this property: Electricity can conduct through carbon, but carbon does have a significant resistance, and much of the electrical energy will be lost as heat energy when it passes through carbon. Not coincidentally, carbon happens to be used frequently to make resistors.
The electrical conductivity of an element is related to how many valence electrons an atom of that element has. An atom is orbited by electrons, but not all of these electrons orbit in the same place; there are different layers of space surrounding the nucleus of an atom in which electrons can exist. Each of these spaces is called a shell. Usually, electrical engineers are most interested in the outermost shell of atoms. The outermost electron shell is called the valence shell, and electrons which exist in the valence shell are called valence electrons.
Once again, it is no coincidence that the semiconductor elements all have the same number of valence electrons: Four. The common semiconductor elements are carbon (C), silicon (Si), germanium (Ge), and antimony (Sb). All of these are "Group IV" elements, meaning they have four valence electrons.
When people talk about "semiconductors" in the PC era, they are usually talking about silicon. Silicon is the most common element on planet Earth (about 28% of all matter on the planet is silicon), so it's relatively easy to come by, but for chip-making purposes, it's not as cheap as you might expect: The silicon used in chips must be super-refined.
In any case, in a basic configuration of pure silicon, the silicon atoms actually form a very tidy grid. The silicon atoms share covalent bonds, which are chemical bonds formed when two atoms share two electrons with each other (i.e. one electron from one atom enters the valence shell of the other atom, and one electron from the other atom enters the valence shell of the first atom). Each silicon atom, then, is bonded to four others: One above it, one below it, one to the left, and one to the right. Each atom has eight electrons around it (because each bond adds an additional electron to the atom's valence shell).
What makes semiconductors like silicon particularly useful is how readily their conductivity can be changed by doping. On its own, silicon is only a mediocre conductor, but the conductivity of it can be changed quite significantly by doping, which is the process of embedding a very small amount of atoms of other elements within the silicon. Typically, only about one in a million silicon atoms are replaced with atoms of some other element, but that one in a million makes a surprisingly big difference.
Even more interesting is how doped silicon responds to the application of electrical voltage.
It turns out that by applying different electric charges to doped silicon, you can actually change how conductive or resistive that silicon is. It is this effect which made the invention of the transistor possible.
So yes Pat is right the silicone and semi-condutors are 'proactive and reacts to the environement.
D'ont thanK me thank goes to
http://www.geocities.com/SiliconValley/2072/index.html